Solution-phase electrochromic devices and various applications thereof are described in, e.g., U.S. Pat. Nos. 4,902,108 (the '108 Patent), 3,806,229, and 3,451,741; U.S. patent application Ser. Nos. 515,511, filed Apr. 30, 1990, and 720,170 and 720,177, both filed Jun. 25, 1991, all three of which are commonly owned with the present Application; European Patent Application Publication Nos. 0 012 419, 0 430 684, 0 430,686, and 0 435 689; and Chang, "Electrochromic and Electrochemichromic Materials and Phenomena," in Kmetz and von Willisen, eds., Non-emissive Electrooptic Displays, Pergamon Press, New York, New York, USA, pp. 155-196 (1976). All of these patents, patent applications and articles are incorporated herein by reference.
In typical solution-phase electrochromic devices, and particularly such devices which are single-compartment and self-erasing, a solution is held in a thin layer in a compartment formed by two walls, which are typically of glass or clear plastic and at least one of which (including the electrode layer described below) is transparent to light (electromagnetic radiation of wavelength in the visible range), and by spacers or sealant between, and holding apart, the two walls and forming the periphery of the compartment holding the solution. The sides, which face each other and are in contact with the solution, of the two walls are each coated with an electrode layer, which is a layer of material, such as tin oxide, tin-doped indium oxide, indium tin oxide, fluorine-doped tin oxide, gold, rhodium or the like, which is electrically conductive and forms an electrode in contact with the solution. When a sufficient potential difference is applied between the electrode layers, across the solution, the transmittance of the solution changes at at least one wavelength in the visible range and, as a consequence, the solution changes color, becomes darker, or becomes clearer. Typically, the solution in such a device will be clear or slightly colored (tinted) in its zero-potential, equilibrium state and will be darkened through electrochemical reaction(s) when a potential difference is applied. In a solution-phase device, the electrochromic compounds (those which have a change in transmittance in the visible wavelength range upon electrochemical oxidation (anodic electrochromic compound) or reduction (cathodic electrochromic compound) remain in solution upon oxidation or reduction in operation of the device. In the preferred single-compartment, self-erasing devices, there is one solution compartment, both anodic and cathodic electrochromic compounds are together in the same solution and free to diffuse throughout the entire solution, and self-erasing occurs, when there is no potential difference between the electrode layers, as oxidized anodic compound reacts with reduced cathodic compound to return both to their zero-potential equilibrium states. Solutions of variable transmittance in solution-phase electrochromic devices may comprise components in addition to solvent and electrochromic compounds. Such components may include inert, current-carrying electrolyte; thickening or gelling agents such as polymethylmethacrylate; and UV-stabilizing agents, which are compounds which inhibit degradation of the electrochromic compounds upon exposure to ultraviolet (UV) radiation.
In practical applications, such as dimmable rearview mirrors for motor vehicles, variable transmittance windows, or display devices, the electrochromic compounds and other components in the solutions of variable transmittance in solution-phase electrochromic devices must be stable to a range of environmental conditions (e.g., temperature variations, changes in intensity of UV radiation) and over a large number, typically on the order of at least 10,000 to 100,000, cycles of darkening and clearing (i.e., applying and removing electrochemical potential), which also may occur over a range of environmental conditions.
Generally, adding a component, particularly one which consists of a potentially reactive organic compound, to the solution of variable transmittance of a solution-phase electrochromic device, destabilizes the device to variations in environmental conditions and cycling between dark and clear states. This destabilization may occur, for example, on account of chemical reactions of the added component, with oxidized anodic or reduced cathodic electrochromic compound, that result in degradation of the electrochromic compound. Such destabilization generally disadvantageously reduces the cycle life of a solution-phase electrochromic device, i.e., the number of times the device can be varied in transmittance to light by application of a potential between the electrodes and returning the potential (across the solution) to zero or near zero and retain, in the cycling, an acceptable change in transmittance. Reduction in the cycle life of an electrochromic device reduces, and may eliminate, the practical value and commercial acceptability of the device, the solution of variable transmittance employed in the device, or apparatus, which employ the device as a component of variable transmittance.
Some commercially available, sideview or outside rearview mirrors for use on motor vehicles have a blue tint (a slightly blue coloration). The associated, enhanced reflectance on the blue end of the visible light spectrum, as compared to the green, yellow and red portions of the spectrum, has been purported to improve rear vision at night, because the spectral sensitivity of the human eye is somewhat shifted to the blue end of the spectrum on going from day (photopic) vision to night (scotopic) vision. Blue-tinted mirrors, as compared to achromatic mirrors, may have some effect on glare from headlights of following vehicles at night since these mirrors tend to reflect less of the longer wavelength green, yellow and red light that predominates in most motor vehicle headlights and provide better contrast by enhanced reflectance of the shorter wavelength blue light which is more abundant in reflected moonlight and certain types of street lights than in headlights. In addition, since blue tinted mirrors have generally appeared on luxury vehicles, the blue tint of the mirrors has come to be associated with luxury or elegance.
With electrochromic, dimmable, outside rearview mirrors, the concern for glare from following-vehicle headlights is significantly diminished, because the reflectance of the electrochromic mirror can be reduced to levels where little or no glare is perceived by drivers during night driving. However, presumably because of the luxury or elegance factor associated with conventional (non-electrochromic) rearview motor vehicle mirrors, the desire for a blue tint in the high reflectance state of the dimmable, electrochromic mirrors, especially those to be used outside the vehicle, has remained.
The blue tint of conventional mirrors can be obtained by using colored coatings in contact with the reflector or by placing interference coatings, which reflect predominantly blue light, on a flat substrate like a sheet of glass. Similar types of colored or interference coatings could be placed somewhere in the structure of an electrochromic rearview mirror, but this would be undesirable because it would add cost and complicate manufacturing.
A problem in the art, then, has been to find a way to conveniently add desirable tints (slight colorations) to solution-phase electrochromic devices in their zero-potential, equilibrium states. With such devices used as variable transmittance components of variable reflectance ("dimmable") rearview mirrors for motor vehicles, a desired tint is blue. With such devices employed as the variable transmittance components in other types of variable transmittance or variable reflectance apparatus, tints of colors other than blue (e.g., yellow) are sometimes desirable.